1. Technical Field
The present invention relates to apparatus and methods for analysis of blood samples in general, and for the determination of the hematocrit of a blood sample in particular.
2. Background Information
Physicians, veterinarians and scientists have examined human and animals' biologic fluids, especially blood, in order to determine constituent particulate quantities as well as to identify the presence of unusual particulates not seen in healthy subjects. The particulates generally measured, quantified and identified include red blood cells (RBCs), white blood cells (WBCs), and platelets. RBC analyses can include determinations of RBC number, size, volume, shape, hemoglobin content and concentration, and the hematocrit (also referred to as the packed cell volume). RBC analyses can also involve determining the presence and/or concentration of certain components within the red blood cells such as DNA, RNA, including the detection of the presence and/or enumeration of hematoparasites (e.g., malarial parasites) either in the RBCs or trypanosomes which are extracellular or leishmaniasis organisms which are in the WBCs as well as many other hematoparasites. WBC analyses can include a determination of the population frequency of WBC sub types generally referred to as a differential WBC count, as well as the notification of any unusual cell types not found in healthy subjects. Platelet (or in certain animals including birds, reptiles and fish, thrombocytes which are similar in function to platelets in mammals but are about ten times larger and nucleated) analyses can include platelet number, size, shape texture, and volume determinations, including determining the presence of clumps of platelets or thrombocytes within the sample.
Known blood examination techniques, described in detail medical texts such as Wintrobe's Clinical Hematology 12th Edition, generally divides the examination methods into manual, centrifugal, and impedance type methods. Manual methods for cell enumeration typically involve the creation of an accurately determined volume of a blood or fluid sample that is quantitatively diluted and visually counted in a counting chamber. Manual examination methods include examining a peripheral smear where the relative amounts of the particulate types are determined by visual inspection. Centrifugal examination methods involve centrifuging the sample, causing the sample to separate into constituent layers according to the relative densities of the constituents. Each component layer can be stained to enhance visibility or detection Impedance methods involve the examination of an accurate amount of blood which is treated according to the particulate being measured; e.g., lysing RBCs for enumeration of the nucleated cells and volumetrically diluting the sample in a conductive fluid. The process typically involves monitoring a current or voltage applied to sample passing through a narrow passage to determine the effect particles have on the current/voltage as the particles pass through in single file. Other techniques involve analyzing the intensity and angle of scatter of light incident to particulates passing single file through a light beam. Flow cytometric methods can also be used that involve staining particulates of interest in suspension with fluorophores, exciting the stained particulates with light of appropriate wavelengths, and analyzing the emission of the individual particulates/cells.
All of the aforementioned methods, other than the peripheral smear or centrifugal separation, require dispensing a precise volume of sample. Inaccuracies in the sample volume will result in quantitative errors of the same magnitude in the associated analysis. With the exception of centrifugal methods, all of the aforementioned methods also require the sample to be mixed with one or more liquid reagents or diluents, and also require calibration of the instrument to obtain accurate results. In the case of peripheral smears, a high degree of training is needed to properly examine the smear. A number of the aforementioned methods generate large volumes of contaminated waste which is expensive to handle. Additionally, the above-described methods are not suitable to determine the complete blood count (CBC) in birds, reptiles, fish and certain mammals where the red blood cells size is very small.
Despite the complex amount of hematologic information obtained from the complete blood count, one test is most often needed: the hematocrit. It is the hematocrit that tells the physician whether the patient is anemic due to bleeding or nutritional causes such as the relatively common iron deficiency in growing children and women of reproductive age, associated disease processes such as chronic infections, metabolic disease such as uremia or neoplastic illnesses as well as pharmacologic effects. An elevated hematocrit indicates the presence of too many red blood cells due to disease processes such as dehydration where the blood is concentrated. An elevated hematocrit can also be indicative of true increases in red blood cell mass that occur as a result of disease processes such as polycythemia, or pharmacologic effects such as the administration of too much anabolic steroids or chronic hypoxia due to lung disease or certain types of congenital heart disease. The importance and utility of the hematocrit make it one of the most frequently requested tests performed on blood. Consequently, easy, accurate, inexpensive and rapidly available hematocrit determinations are highly desirable and will benefit patients. An instrument that can use a disposable analysis chamber, one that can operate with no internal fluidics other than capillary flow (i.e., one that is operable independent of gravity and orientation), and one that can be utilized as a handheld device would be a great benefit.